Interestingly, the Atlantean effects also activate if they are detached from an Xyz Monster (the effect only requires the monster to be sent to the Graveyard, like "Dandylion"), so a great addition to this deck are WATER Xyz Monsters, like "Number 17: Leviathan Dragon". Here is a list of WATER Xyz Monsters that can Trigger the Atlantean's effects (note that all current Atlantean that share this effect are Level 2, 3 or 4. However, you can use cards like Forgotten Capital Lemuria for Xyz summoning some of these higher Rank monsters):

The spell card of this archetype, Atlantean Roar, allows you to summon up to 3 Level 3 or lower Sea Serpent type monsters from your graveyard, but you can't special summon the turn it is activated, and there must be at least 3 acceptable monsters in your graveyard. Since most of the effects of Atlantean activate when they are sent to the Graveyard, the later shouldn't be so hard, but cards like Skreech can help you dump cards. Atlantean Roar is a Quick-Play Spell, you can activate it during your opponent's End Phase, so you can Special Summon normally in your turn, as long as it was set before.

Atlantean don't have much attack power by themselves, so decks run with cards to clean the field. Genex Undine can destroy any card your opponent controls by sending either Atlantean Marksman or Atlantean Armed Soldier, while adding a Level 3 Tuner to your hand. Abyss Soldier and Abyss Warrior can trigger the effect of the Atlanteans while returning a monster to the hand and a card in the Graveyard to the top or bottom of the deck, respectively.

One of the great support cards for this deck, Deep Sea Diva, can Special Summon any low-level Atlantean monster from your deck. If you choose to summon Atlantean Marksman, every time you inflict battle damage to your opponent, you can Special Summon another Atlantean monster from your deck, like Atlantean Assaultman for a beatstick, while also having monsters for a Rank 3 Xyz Monsters, or Synchro material for Level 4 Synchro Monsters or above.

There are many choices for Synchro Monsters. You can go for Ally of Justice Catastor (Diva + Marksman or Assaultman), Brionac, Dragon of the Ice Barrier (Diva + Dragon Rider), Gungnir, Dragon of the Ice Barrier (Diva + Armed Soldier + Marksman) and any other of your choice. The Ice Barrier Dragons discard cards to activate their effects, wich clean the field by themselves. Brionac can return any card on the field to the hand any number of times in your turn, so you can use its effect to return one Atlantean monster in your field, then discard it to return another one to the hand, and since it was discarded, its effect activates.